How does displacement relate to HP

[azopelnut]

I have a general question about how boring cylinders relates to power gains.
The engine is not an Opel engine but certainly the question could just as easily apply.
I acquired an abandoned 75 X19 with a 79 motor and I am in the process of rebuilding the 1500cc engine.
The engine was froze and upon inspection all but one of the cylinder walls were good. I am intending to reused the pistons and sleeve the one bad cylinder.

[/U][/B]QUESTION[/B][/U]
If I were to bore out all cylinders to 87mm that is a .60mm overbore.
How does this alone equate to power improvement on a HP or percent basis? Is there a formula? Obviously I am trying to keep costs down and new pistons and a rebore of four cylinders is more expensive than sleeving one cylinder and reusing stock pistons.

[RallyBob]

Originally Posted by azopelnut If I were to bore out all cylinders to 87mm that is a .60mm overbore. How does this alone equate to power improvement on a HP or percent basis? Is there a formula? Obviously I am trying to keep costs down and new pistons and a rebore of four cylinders is more expensive than sleeving one cylinder and reusing stock pistons.

The 'old school' rule of thumb was about 4% more hp and torque when enlarging an inline 4-cylinder 100 cc's. But as we both know, it's more complex than that. 100 cc's is more percentage-wise on a 1500 vs. 1900. And it depends on the baseline power as well. But in general, unless you weaken the engine by overboring, you'll always make more power, not less.

From experience, the best thing you can do for that 1500 engine is increase the intake airflow. On a flowbench, the exhaust ports actually surpass the intake ports in flow. I've used Toyota 2TC intake valves on a 1500 Fiat, and got the intake flow from 62 cfm to 78 cfm (+25%) without too much trouble. The exhaust flowed 67 cfm BTW...

Bob

[azopelnut]

Thanks Bob
according to my calculations
bore 86.4 x stroke 63.9 = 1498cc
bore 87x stroke 63.9 = 1519cc
net gain is only 21cc or 1.4% increase

I think I will sleeve it and save the money for my Opel projects

[baz]

i had an 1300 X1/9 for a while (about 2 years ) so as you do i played with it

never got bored revving it up to 8500 /min

so after a time i looked at the 1500 as an upgrade , then decided it was not that good of an upgrade as it could not rev as much so kept on going through the FIAT engines till i found some thing worth while

well it turned out that with a little work ( as you know i consider a body to floor swop as little ) i found that the strada 130T arbarth was a nice fit with the 1500 hood

now this engine is waaayyyyyy nice and lots of bits from arbarth to make you go broke

i dont know if the strada sold in the states but the motor came in the 131 , 132 , lancia's and is a nice twinky

Google Image Result for http://strada130.com/images/130-5.jpg

well worth a look for one but do and i mean do work on the brakes if you go for one